1. Distillation principle
The basic principle of distillation is the partial gasification and partial condensation of the liquid mixture several times, using the different volatility of the components (relative volatility, α) of the characteristics of the unit operation to achieve the purpose of separation. Distillation can be classified according to its operation method: simple distillation, flash distillation, distillation and special distillation, etc.
2. Brief description of the process
The vapour enters from the bottom of the tower. Evaporation of the gas phase and down liquid counter-current contact, two-phase contact, down liquid in the easy to volatile (low boiling point) components continue to transfer to the gas phase, gas phase in the difficult to volatile (high boiling point) components continue to transfer to the down liquid, the closer the gas phase to the top of the tower, the higher the concentration of its easy to volatile components, and down liquid closer to the bottom of the tower, the more difficult to volatile components are enriched, so as to achieve the purpose of component separation. The gas phase rising from the top of the tower enters the
Condenser, and part of the condensed liquid returns to the top of the tower as reflux into the distillation column, while the rest is taken out as distillate. The liquid flowing from the bottom of the tower, part of which is sent to the reboiler, heated and evaporated into the gas phase to return to the tower, and the other part of the liquid is taken out as kettle residue.
3. Problems
However, throughout the chemical process, it is difficult to achieve fine temperature control of the entire condensing system. This can cause some substances as well as impurities to exhibit physical and chemical properties that are difficult to distinguish. That is, the system needs to be inevitably mixed with other impurities, there is no doubt that the purpose of the entire distillation separation process is greatly reduced. For this situation, the actual production is often taken to enhance the system temperature to solve the problem, but on the other hand, the higher the temperature is, the more useful substances will be difficult to present a complete condensation reflux situation, and then the separation effect of the yield decline.
4. Solution
According to the principle of distillation and the analysis of the process, the fine degree of temperature control determines the effectiveness of distillation. In the actual implementation of the production process, variable factors and influencing factors are objective. In addition, there are many variables that need to be controlled and the combination of variables can lead to results that deviate from the desired target, so a fine and accurate control of the conditions of the automated system plays an important role in both the efficiency of the process and the overall quality system.
Automatic control
1. Concept
The use of external equipment or devices to make a machine, equipment or production process operate automatically according to a predetermined pattern of operation or parameters without direct human involvement. Automatic control is relative to manual control.
2. Application
Feedback adjustment is the core of automatic control, for example, temperature automatic control is the temperature as an output quantity, used to regulate heating and cooling, so as to achieve a constant temperature. Automatic control is used to regulate the temperature of the distillation column, which can greatly improve the efficiency and quality of distillation.
The temperature of the condensing system at the top of the distillation column is monitored by a temperature control system with sensitive temperature sensors installed in the system, which change as the components at the top of the distillation column change. The system sets a certain monitoring temperature line, if the system temperature is abnormally above the limit, the automatic monitoring and temperature control system will automatically adjust, reset the temperature and change the appropriate component reflux ratio to quickly solve if the
Distillation Tower end of the steam system temperature exceeds the control range, that is, the system of uncondensed material ratio increases, then the total system quickly change the temperature, and reduce the reflux ratio. And vice versa, the automatic control system regulates the system to ensure that the distillation process is always carried out at a constant and suitable condition.
In this way, the operation of the condenser control is fully monitored by an automated process, which avoids the need for manual changes to the system temperature and reflux ratio, thus optimising the quality of the product.
Distillation columns and automation
1. Gas-liquid balance under automatic control
The essence of the operation of the distillation process is the expression of three chemical equilibria, namely the material, phase and thermal equilibria, which are closely interlinked. For example, when the steam pressure is changed, the actual evaporation volume at the bottom of the tower is quickly changed, which directly leads to a thermal non-equilibrium in the system and an imbalance in the gas-liquid phase state. Therefore, the key to the production process is the equilibrium of heat and steam volume. Under other constant conditions, the flow rate is proportional to the evaporation volume and the tower difference of the distillation tower, according to which the evaporation volume can be controlled by changing the pressure at both ends of the distillation tower. The automatic system process takes the feed volume and the tower kettle pressure in series, the electronic signal of the tower bottom pressure is transmitted to the flow regulating valve system, which will automatically adjust the flow rate. The real time optimum change of the feed volume can achieve the relative stability of the kettle pressure, avoiding the final separation effect of the whole process by the fluctuation of the feed volume.
2. Temperature control of the distillation column
(1) distillation section temperature control sensitive plate taken in the distillation section of a layer of the tower plate, called distillation section temperature control. Applicable to the top of the tower product quality requirements or gas phase feeding occasions. Adjustment means is according to the sensitive plate temperature, appropriate adjustment of reflux ratio. For example, when the sensitive plate temperature increases, it reflects the top of the tower product composition Zn down, so this time issued a signal to increase the reflux ratio, so that XD rise to the qualified value, the sensitive plate temperature down to the specified value.
(2) distillation section temperature control sensitive plate taken in the distillation section of a layer of the tower plate, called distillation section temperature control. Applicable to the tower bottom product requirements of high occasions or liquid phase feed, its use of regulation means according to the sensitive plate temperature, appropriate adjustment reboiler heating. For example, when the sensitive plate temperature drops, it reflects that the liquid phase composition Xw at the bottom of the kettle becomes larger, and the product at the bottom of the kettle is not qualified, so a signal is sent to increase the heating of the reboiler appropriately, so that the kettle temperature rises in order to maintain the specified value of work.
(3) Temperature difference control When the boiling point of each composition in the raw material liquid is similar, and the purity requirement of the product is high, it is not suitable to use the general temperature control method, but should use the temperature difference control method. Temperature control is based on the principle that the temperature variation of two plates is always much larger than the temperature variation on a single plate to design, using this method is easy to ensure the purity of the product, but also conducive to the selection and use of instruments.
3. Application examples
The most critical technology lies in the fine control of the temperature of the condenser system at the top of the distillation tower, which is the bottleneck of the entire PVC industrial production process. With the development of technology, automatic control systems are fitted to the condensing section of the PVC distillation process, which has obvious advantages over conventional means in terms of actual data.
a. VC low boiling tower system
The VC low boiling system is used to separate components such as acetylene which have a lower boiling point than VC. In practice, the actual purpose of the low boiling analysis cannot be efficiently achieved because the two are often more identical in some properties and the acetylene component of the liquid phase can be miscible with the VC liquid phase system. In accordance with the actual situation, a computerised automated control system is installed in the condenser system. It enables real-time monitoring of the temperature at the top of the distillation column and automatic adjustment according to the actual situation to achieve precise control of the condensing system temperature and avoid abnormalities such as too high or too low. The automatic temperature control can then be used to optimise the return ratio and flow rate at the top of the distillation column.
b. VC high boiling tower system
The fraction VC liquid from the low boiling tower is then separated from the high boiling point material by the distillation process in the high boiling tower system, and the high purity gaseous phase VC is converted into finished distillate from the top of the distillation tower through the condensing system. In practice, the traditional condensing system often fails to achieve fine control of the temperature, which ultimately affects the quality of the product. Once the high boiling tower condensing system is equipped with a computerised automatic temperature system, the computer monitors the temperature of the top of the tower and the condensing system in real time, automatically controlling the return flow and return ratio values to maximise the efficiency of the whole process.
New trends
Supergravity distillation columns
In recent years, the emergence of supergravity distillation technology, the use of high-speed rotation generated hundreds to thousands of times the gravity of the supergravity field instead of the conventional gravity field, greatly enhance the gas-liquid transfer process, the transfer unit height is reduced by 1 order of magnitude. This transforms huge tower equipment into a supergravity distillation machine of less than 2 metres in height, achieving increased efficiency and reduced size. Supergravity distillation changes the traditional mode of distillation in a tower plant and allows for a continuous distillation process as long as the distillation is carried out in an indoor plant. For the development of society can save steel resources, extend the life of the earth's resources; for the development of enterprises, can save site and space resources, reduce pollution emissions, improve product quality, improve business management mode, reduce production labour intensity, increase the safety of production.
The combination of super gravity distillation tower and automatic control can improve the distillation efficiency of the distillation tower in both gravity field and temperature at the same time, which greatly promotes the industrial development.
Source: Reprint
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